1. Field of the Invention
Embodiments of the invention generally relate to a method for identifying electromagnetic fields with implantable medical devices, specifically electromagnetic fields that occur when using nuclear magnetic resonance imaging and tomography (e.g., MRI, MRT) devices.
2. Description of the Related Art
Although magnetic resonance (MR), magnetic resonance imaging (MRI) and magnetic resonance tomography (MRT) examinations (hereinafter collectively referred to as MR) are generally becoming ever more significant in diagnostic medicine, MR examinations are contraindicated for some patients. Typically, such a contraindication may be caused by an active implanted medical device.
Generally, besides possible heating effects of implants, particularly with small conductive structures, other problems include erroneous identifications of events in the heart, such as, but not exclusively, ventricular fibrillation, or fast cardiac dysrhythmia, (VF) and the high static magnetic fields and resultant magnetizations of electric components. The above-mentioned effects typically occur particularly with active implants, such as, but not exclusively, defibrillators/cardioverters (ICDs), pacemakers, cardiac resynchronization devices, neurostimulators or drugs pumps, but also with passive implants, such as monitoring devices. However, generally, the function of other implanted medical devices may also be disturbed by electromagnetic fields, and these devices must not be operated in environments subject to increased electromagnetic loads. The following prior art concerns, in particular, the problem of detecting electromagnetic interference fields in the presence of implanted medical devices (IMDs).
U.S. Pat. No. 6,522,920 to Silvian et al., entitled “System and Method of Protecting Transformer-Driven Switches from External Magnetic Fields”, describes a system for protecting the inductively actuated high-voltage switch of an ICD during shock delivery in a magnetic field. In Silvian et al., the system monitors whether a sufficient gate voltage is provided at the moment of the shock delivery. According to Silvian et al., the monitoring is assessed by means of an assessment of the secondary voltage of the inductive drivers.
For example, United States Patent Publication 20080071168, to Gauglitz et al., entitled “Systems and Methods for Sensing External Magnetic Fields in Implantable Medical Devices”, provides an impedance measuring unit and an RLC member for detection of a magnetic field and to perform the detection by determining the inductance in the RLC component. Typically, an additional or adapted impedance measuring unit is required for this purpose.
U.S. Pat. No. 7,509,167 to Stessman, entitled “MRI Detector for Implantable Medical Device”, describes the identification of a magnetic field by the measurement of the timings for the actuation of the high-voltage transformer of the primary side or alternatively by the measurement of peak currents during a charging cycle. For example, the disadvantages of the described system of Stessman are that the measurement of the timings has to be very quick (us to ns), and that a direct current measurement may only be implemented with difficulty due to the necessary measuring resistor, which would lengthen the charge times for the high-voltage capacitor.
Furthermore, U.S. Pat. No. 6,101,417, to Vogel et al., entitled “Implantable Electrical Device Incorporating a Magnetoresistive Magnetic Field Sensor”, appears to describe the use of a giant magnetoresistive ratio (GMR) sensor instead of a reed switch, however not for detection of MR fields, rather for activation of a magnet mode in the implant.
Generally, the technical solutions provided in the prior art above, for detecting electromagnetic interference fields with implanted medical devices, include uncertainties regarding the sensitivity or specificity, or such solutions are not efficient enough.
Therefore, in view of the above, there is a need for an efficient and reliable detector for electromagnetic fields for MR detection.